1. Field of the Invention
The invention described herein relates to pellet inspection apparatus, and more particularly, to equipment designed to automatically inspect nuclear fuel pellets to determine whether the pellets meet length and end squareness standards.
2. Description of the Prior Art
Nuclear fuel pellets used in many nuclear reactors are made of enriched uranium dioxide and are right circular cylinders measuring about 0.366 inch diameter by 0.600 inch length. During the manufacturing operation, after pellets are fired, ground and inspected for flaws on their outer surface in the manner disclosed in copending application Ser. No. 646,789, filed Jan. 6, 1976 and entitled "Automatic Inspection And Sorting System For Chipped Nuclear Fuel Pellets", they are transferred to a second inspection area where length and squareness measurements are taken. Criteria have been established which require that each pellet end must be held to a high degree of squareness, e.g., 0.004 in. with the pellet axis to help assure loading of the pellets in fuel tubes without jamming. This production operation is carried out by locating the pellet in a V-block with its axis vertical to a surface plate and positioning a dial indicator on the top surface of the pellet. As the pellet is rotated in the V-block relative to the dial indicator, the degree of nonsquareness is observed and the pellet discarded in the event it exceeds predetermined limits. The major disadvantages of this measuring technique are that it is slow and involves very tedious work, pellet chips on the top surface interfere with the measurements, the inspector must observe both the high and low readings and then calculate the difference therebetween to determine pellet acceptance.
The pellet length is measured to determine pellet volume and density and is accomplished by positioning the pellet in a conventional micrometer which provides a direct reading of the pellet length. Since planes passing through the ends of the pellets are rarely parallel, the length measurement attained is usually longer than the mean pellet length along the axis, upon which volume calculations are based. With a 0.004 in. nonsquareness tolerance on each end, the error contribution can range from 0 to about -0.67% absolute density.
Further, the manual methods used in determining length and end squareness of uranium dioxide pellets cannot reasonably be used with plutonium enriched pellets because of the extreme health hazards which result from their radioactivity and toxicity characteristics.